A device for gauging the level of a liquid in a container comprises a transmitter for transmitting a microwave signal towards the surface of the liquid, a receiver for receiving the microwave signal reflected against the surface of the liquid, and a signal processing device for calculating the level of the liquid in the container from the propagation time of the transmitted and reflected microwave signal.
Such device has become more and more important, particularly for petroleum products such as crude oil and products manufactured from it. By containers is here meant large containers constituting parts of the total loading volume of a tanker, or even larger usually circular-cylindrical land-based tanks with volumes of tens or thousands of cubic meters.
In one particular kind of radar-based device for gauging the level of a liquid in a container the microwave signal is transmitted, reflected and received through a vertical steel tube mounted within the container, which acts as a waveguide for the microwaves. An example of such tube-based level gauge is disclosed in U.S. Pat. No. 5,136,299 to Edvardsson. The velocity of microwaves in a waveguide is lower than that for free wave propagation, but in the calculation of the level of the liquid in the container from the propagation time, this may be taken into account either by means of calculations based on knowledge of the dimensions of the waveguide or by means of calibration procedures.
Further, the gas above the surface of the liquid reduces the velocity of the microwaves. This velocity reduction may be accurately estimated, but only if the gas composition, temperature and pressure are known, which hardly is the case.